Give Yourself Some Space
Use a longer needle that you ordinarily would. Linen stitches won’t compress the the way wool does. You won't be able to just squish on some more stitches and still work comfortably.
Wood or Bamboo needles will be more grippy and your stitches won’t slip and slide around so much. You tension will be more even if you’re not struggling to keep the stitches on the needle.
Join At The Edges
Keep an eye on what’s left in your ball. Plan to join on new balls at the edges or in some other inconspicuous spot.
Weave in ends using duplicate stitch. Check out this tutorial from CoCo Knits for more information.
Try a Braided Join
Try joining on new yarn with the braided join. Done on an edge, this join will be nearly invisible. See a tutorial of the Braided Join technique from La Visch Designs here.
Don’t bother. Ribbing will have zero elasticity in linen. Focus on patterns that have lots of lace, cables or texture. Look for edge treatments like garter stitch edges, turned hems, or rolled edges.
Win the Ball Game
Stick something in the center of your ball to keep the core stable (a paper towel roll, perhaps) and work from the outside.
You’ll get the most accurate results if you measure your swatch after it has been washed and dried the same way that you’ll wash and dry the garment.
You can go a couple of ways with linen and have good results. Machine wash cold and tumble dry. Linen will come out of the dryer a little crunchy, but it will not shrink. You can quell the crunch by ironing with steam. Alternately, machine wash cold, tumble dry for 15 minutes, then lay flat. Linen loves being washed and used. The fabric will become softer and more drapey as it is used and washed.
If you’re used to working with wool, there’ll be a learning curve for tensioning linen. Linen is inelastic. Holding it tightly is the source of the hand pain many complain about after using linen. Practice tensioning the yarn without pulling it too tightly while you work your swatch. Your hands will thank you.